Sirian is a hard science-fiction setting, focused on space habitation and exploration.
What's 'hard sci-fi' mean? If fiction is a scale from soft to hard, then on the soft side you have fantasy, on the hard you have the most scientifically accurate setting you can imagine. Somewhere in between is where most settings live. For instance, this puts Lord of the Rings firmly on the soft side. Similarly, Star Wars is fairly soft, despite being sci-fi. Star Trek a little closer to the hard side of the spectrum. Space Above and Beyond, Firefly, or Battlestar Galactica (the new series) are much more Hard Sci-Fi beyond which is nearly anything written by Asimov.
Sirian is aiming for somewhere around the BSG to Asimov end of the spectrum, meaning it's fairly scientifically realistic based on what we currently know of science and technology.
Why do we define this here? Unlike fantasy, there isn't a 'default' science fiction setting. In fantasy, you can say "I'm running a fantasy game," and people typically have a rough idea what that means: a medieval or Renaissance Western European model, with Elves Dwarves Dragons and magic.
If you say "I'm running a science-fiction game," there is no 'standard'. Do you mean Star Wars (which is almost more fantasy than sci-fi) or cyberpunk (near future, realistic technology)? Are you talking space travel or a planetary dystopia? Even if you narrow it down to space, are you thinking starship battles or interplanetary gates? This isn't a genre that has a 'default' setting - so we have to define one carefully to get everyone on the same page.
Given that we already have a fantasy setting in Vaxia, our target for Sirian is hard sci-fi in order to give players a real variety of tone to choose from.